PROJECT SUMMARY Role of BRCA1 in normal and neoplastic bone marrow cells We have recently discovered that Brca1 is necessary for normal hematopoiesis, but mutation of this gene in hematopoietic cells rarely predispose carriers to leukemia. We have collected a number of convergent data sets that serve as the foundation for the studies proposed here. First, we have found that humans heterozygous for BRCA1 mutations may have an increased risk for chemotherapy-associated febrile neutropenia. Second, mice deficient for Brca1 in the hematopoietic system experience bone marrow failure associated with severe hematopoietic stem cell and progenitor defects, and third, mice heterozygous for Brca1 deficiency have slight defects in bone marrow reconstitution due to problematic functional hematopoietic stem cell activity. From these data, we have developed two hypotheses that we propose to test: BRCA1 is required for normal hematopoiesis and BRCA1 plays a role in emergency granulopoiesis, which is the acute response of hematopoietic progenitors to infection or other stressors. We propose that BRCA1 is regulated by interferon regulatory factor 8 (IRF8) expression, which is induced by infections or other stresses that lead to emergency granulopoiesis. Interestingly, an absolute requirement of BRCA1 for hematopoiesis may explain why people with BRCA1 mutations do not have an increased risk for leukemia: their bone marrow stem and progenitor cells die without BRCA1 before these cells have a chance to transform. Here, we will first identify the cellular and molecular events required for Brca1 to maintain normal hematopoiesis and examine the phenotypes of different human BRCA1 mutations using a humanized Brca1 mouse model. We will also conduct a study with additional patients from our cancer genetics clinic to examine the relationship between BRCA1 mutations and hematopoietic toxicity. Finally, we will examine the increases in BRCA1 levels during stress granulopoiesis and BRCA1 function in this pathway. This area of investigation is unexplored, and our results will facilitate a better understanding of the requirements for Brca1 in normal and neoplastic hematopoiesis and the chemotherapy toxicities in patients, ultimately leading to improved treatment of hereditary breast and ovarian cancer syndrome patients as well as insight into the tissue specificity of BRCA1 mutation-associated tumorigenesis.